The invention relates to a method and a device for image coding having a particular, but not necessarily an exclusive, application in the transmission of video signals, for example in video telephone systems.
In order to obtain good quality signals for such use, the video signal is converted into a digital signal. The format of the digital signal is specified by the International Consultative Committee for Telegraphs and Telephones (CCITT) H.261 standard. However, the method of digitisation of the video signal can be selected independently by the user.
The transmission of a digital signal is normally carried out with the aid of a number of circuits which are frequently divided into a video input signal section, a coder core and a video multiplexer (VMUX). The video input signal section effects suitable signal processing of the video input signal. The coder core effects some form of transformation coding of the signal which can be predictive (INTER) or non predictive (INTRA). After the transformation coding, the signal is quantised.
The digital bit stream is then subjected to further code processing by the VMUX in order to effect compression of the bit stream. Traditionally, run length coding, variable length coding and error correction are also effected. The operation of the VMUX is normally effected by means of a signal processor. Since these codings produce different numbers of bits per frame and the bit rate on the transmission line at the output of the image coding is constant, the VMUX contains a buffer store.
Problems experienced with the known technology relate to the prevention of overflow in the buffer store and the buffer store being emptied prematurely. Both of these problems give rise to a deterioration in image quality. At the same time, the content of the buffer store should be as small as possible.
It is an object of the present invention to overcome the foregoing problems by arranging for the buffer store content to be regulated by skipping the coding of a number of flames and/or by setting to zero certain components in the bit stream.